The purity of the silicon wafers depends upon the concentration of different impurities, including heavy metal contaminates (e.g., Fe, Cr, Cu), introduced during the manufacturing and processing of semiconductor devices. The minority carrier lifetime and the diffusion length are used for contamination monitoring in silicon wafers. The challenge is to measure diffusion length, and monitor contamination in the product wafers, at all steps in the processing and manufacturing of integrated circuits.
In current techniques, the intensity-modulated light, with the photon energy larger than the band gap, is directed to the front side of semiconductor. As a result of photo generation, the excess carriers change the surface potential of the semiconductor, and alternative surface photo voltage (SPV) is measured using a transparent conducting electrode placed near the front surface of the silicon wafer, within the illumination area. Diffusion length is determined by measurements of the SPV signals and light fluxes under successive illuminations of the wafer with monochromatic light at different wavelengths.
The American Society for Testing and Materials (ASTM) recommends two methods, F 391 A and B, for SPV measurement of the diffusion length. The calculation of the diffusion length is based upon the solution of the one-dimensional diffusion equation for excess minority carriers, assuming that diffusion length is short compared to ¼ wafer thickness.
This expression is                               Δ          ⁢                                           ⁢          n                =                  Φ          ⁢                                           ⁢                                                    1                -                R                                                              D                  ⁢                                      /                                    ⁢                  L                                +                                  S                  F                                                      ·                                          α                ⁢                                                                   ⁢                L                                                              α                  ⁢                                                                           ⁢                  L                                +                1                                                                        (        1        )            where Δn is the excess minority carrier concentration, L is the diffusion length, α is the absorption coefficient, Φ is the incident light flux, R is the reflectivity of the semiconductor, D is the minority carrier diffusion constant, and SF is the front side surface recombination velocity. This method has been described in the patent to A. M. Goodman in U.S. Pat. No 4,333,051, 1982. The SPV has monotonical dependence versus light flux with linear region for small level excitation. This method has been described in the patent to A. M. Goodman in U.S. Pat. No 4,333,051, 1982.
In the first ASTM-recommended method F391 A, the magnitude of SPV is adjusted to the same value by changing the light intensity at each wavelength. The effective diffusion length is obtained from the linear plot of the light flux, Φ, versus the light penetration depth α−1. The effective diffusion length equals the intercept value LEFF=−α−1 at Φ=0. The effective diffusion length depends on the bulk lifetime, τ, and the surface recombination velocity, Sb, at the back surface of the wafer. If the effective diffusion length is less then one-fourth wafer thickness, LEFF can be taken to be equal to the diffusion length L=√{square root over (D·τ)}, where τ is the minority carrier lifetime.
The second ASTM recommended method F-391-B is the linear constant photon flux method, which uses the SPV measurement for several different wavelengths of light with the same intensity, where the photovoltage has linear dependence versus light intensity. The diffusion length is obtained using the linear plot of the inverse value of the surface photovoltage as a function of light penetration depth. This method is discussed in the patents to Lagowski U.S. Pat. Nos. 5,025,145 and 5,177,351 and J. Lagowski, et. al., Semicond, Sci, Technol. 7, A185 (1992). The apparatus includes halogen light sources with a wavelength selecting wheel for illumination and a quartz disk with indium thin oxide (ITO) film for directing the light onto the wafer surface and detecting an SPV signal.
In the patent to Lagowski et al., U.S. Pat. No. 5,663,657, another SPV probe is used. The SPV electrode consists of a quartz disk with an evaporated transparent conductance indium thin oxide (ITO) film with the diameter smaller than the diameter of the disk and hence the illumination area. The SPV probe configuration allows one to diminish the systematic error of the diffusion length measurement by excluding the influence of the lateral diffusion of the minority carriers in the bulk of the wafer.
In a Russian patent No 2080689 (1994), the apparatus includes a transparent and conductive electrode, a set of light emission diodes and an objective lens to focus the light through said transparent electrode onto a spot of the wafer. The diameter of the electrode is larger than the optical beam diameter. This configuration is different with respect to U.S. Pat. No. 5,663,657, where the illumination area is larger than the electrode and at the same time also eliminates error due to lateral diffusion of the minority carriers in the body of the wafer and provides better spatial resolution for the diffusion length measurement. In Proceedings of 24th ESSDERC'94, Edinburgh, p.601 (1994), using numerical calculations and the experiment, it was shown that this apparatus can be used for fast mapping (2 minutes with 8000 points) of the diffusion length, with improved spatial resolution close to the optical beam diameter, dB, even if L is comparable with dB.